Figure 2. Components of adipose tissue.

a Adipocytes are the main cellular component of adipose tissue, and they are crucial for both energy storage and endocrine activity. The other cell types that are present are precursor cells (including pre-adipocytes), fibroblasts, vascular cells and immune cells, and these cells constitute the stromal vascular fraction of adipose tissue. Vascular cells include both endothelial cells and vascular smooth muscle cells, which are associated with the major blood vessels. The blood vessels in adipose tissue are required for the proper flow of nutrients and oxygen to adipocytes, and they are the conduits that allow for the distribution of adipokines. Vascular cells also secrete, and are responsive to, adipose tissue-secreted proteins. Other active adipose tissue components include macrophages and T cells, which have major roles in determining the immune status of adipose tissue. The fibroblast-derived extracellular matrix functions to provide mechanical support, and excess matrix can lead to adipose tissue dysfunction. Factors that are secreted by these different cellular components are critical for maintaining homeostasis in adipose tissue and throughout the body. b Examples of intercellular communication between different adipose tissue cell types include the counter-regulation between adiponectin and tumour necrosis factor (TNF), and between secreted frizzled-related protein 5 (SFRP5) and WNT5a. Under conditions of obesity the pro-inflammatory factors (TNF and WNT5a) predominate.